| The dwindling of fossil energy and the global warming issues caused by greenhouse gas, CO2, is drawing more and more attentions, which makes the CO2emission reduction a strategic subject for the governments and academic institutions. Source control, reduce or eliminate CO2emissions from the production processes is an effective way for CO2emission reduction. In addition, CO2capture, storage and utilization is another effective way. On one hand, CO2emissions can be reduced by developing more "green" production processes; on the other hand, useful materials could be synthesized from CO2by new technologies. Transformation of CO2into energy sources, raw materials and chemical products, has more far-reaching significance to the future energy structure and source of chemical raw materials. In this paper, to focus the reduction of CO2emissions and utilization of it, the author developed a green electrochemical metallurgy process to produce Fe, Ni and Fe36Ni alloy without CO2emission with new fabricated NilOCullFe inert anode, the anodic behavior and surface oxidation layer formation mechanism of the anode were investigated. A kind of CO2utilization method in molten carbonates were also studied, high-purity ultra-fine carbon powders with high added value were prepared from CO2, the influences of electrolytic conditions to the morphology and structure of the obtained carbon products were systematically investigated; the carbon powders obtained under different electrolytic conditions were assembled into electrodes to test the capacitive performance as supercapacitors. The main research works and study results are as follows:(1) ANilOCullFe anode was tested to be stable in molten Na2CO3-K2CO3at750?for more than600h. During the electrolysis, Fe2O3pellets were used as cathode, the pellets were electrochemical reduced into iron, oxygen ions transferred to anode and discharged there to generate oxygen. Linear sweep voltammetry of wires and graphite rod as anodes were performed to investigate the anodic behaviors of the metal/graphite in this melt. Cyclic voltammetry of wires and NilOCullFe as anodes were also implemented to investigate the formation of the oxidation layer on metal wires/alloy anode. An oxidation layer composed of NiO and NiFe2O4was formed on the surface of the anode which protected the substrate from further oxidation; a layer of copper enriched at the oxides formation/alloy substrate interface, which helps improving the conductivity of the oxidation layer. The surface formations were characterized by XRD, SEM, and EDX. This inert anode promising a free-carbon way for iron production and an effective anode for carbon capture in molten carbonates.(2) Pure iron was electrochemically prepared in molten Na2CO3-K2CO3eutectic melt at750?without green house gas emission using a solid iron oxide pellet cathode and a cheap NilOCullFe alloy inert anode. The detailed reduction kinetics of solid Fe2O3in the melt and also the effect of reduction potential on the carbon content in the iron product were discussed. The reduction mechanism was systematically investigated by cyclic voltammetric measurements, potentiostatic electrolysis combining with the composition and morphology analysis of the products obtained at different potentials. It was found that the reduction of Fe2CO3involves three steps, with the formation of intermediate products, viz., NaFe2O3and NaFeO2. The influence of electrolysis potential/voltage on the carbon content in the products was investigated by using both constant voltage and potentiostatic electrolysis under different conditions. The carbon content was found to be in the range of0.035?0.76wt%, depending on the applied cathodic potential. The iron-based products with higher carbon content can be obtained upon electrolysis at a higher cell voltage or a more negative potential. The optimized cell voltage for production of iron is2.0V, with a current efficiency of93.6%and energy consumption of3.08kWh for production of1kg iron product, the carbon constant in the product is0.11wt%. The present results also demonstrated a controllable extraction of Fe-C steels with desired carbon content through a CO2-free process.(3) Nickel and Fe36Ni alloy were prepared from NiO/NiO&Fe2O3mixed oxides precursors in molten Na2CO3-K2CO3eutectic melts at750?without CO2emission with the NilOCullFe inert anode. The reduction mechanism of NiO was investigated; it was electro-reduced to nickel directly, without any other intermediate products. The optimized cell voltage for production of Ni is1.1V, with a high current efficiency of95.4%and an energy consumption of1.05kWh per kg of Ni. NiO and Fe2O3mixed powders (Fe:Ni=64:36) were pressed into pellets and sintered at800?for2h. Under a selected cell voltage, NiO was reduced into Ni preferentially, and then, alloyed with the reduced Fe to form Fe36Ni. The particle size of reduced alloy is much smaller than the electro-reduced pure metal (Fe or Ni) obtained with the same method. The optimized cell voltage for electro-reduction of Fe36Ni is1.9V with a current efficiency of94.6%and the energy consumption for preparation of1kg of Fe36Ni alloy is2.48kWh.(4) With a nickel cathode and a SnO2inert anode, amorphous carbon powders were successfully prepared by electrochemical conversion of CO2in Li2CO3-Na2CO3-K2CO3eutectic melt in the temperature range of450?to650?under the electrolysis voltages of3.0V to6.0V. The influences of electrolysis temperature and cell voltage on the morphology, structure of the carbon material and energy consumption of the process were systematically investigated. The morphology and particle size of electrolytic carbon was significantly affected by the electrolysis cell voltage and temperature. Different structures including nanoparticle, nanoflake, nanosheet and heart-shape nanostructured cage were observed. The particle size of the obtained carbon was in the range of50nm to2?m, normally decreasing with increasing the cell voltage but increasing with electrolysis temperature. The carbon product with highest BET surface area of868.3m2/g was obtained at450?under5.5V. In general, the current efficiency decreased with increasing the cell voltage and temperature. The optimized energy consumption for production of1kg of carbon products is35.59kWh with a current efficiency of88%at450?under3.5V.(5) Capacitive performances of the amorphous carbon products obtained under different electrolytic conditions were investigated. The electrochemical measurements of cyclic voltammetry (CV) and galvanostatic charge-discharge test in1M H2SO4aqueous solution were applied to test the capacitive performances of the assembled electrodes. Carbon products obtained at higher electrolysis temperature exhibit much poorer capacitance performance. However, the carbon powders obtained at450?exhibit excellent capacitive performance. The carbon powder obtained at450?under4.5V for example, exhibits a specific capacitance of550F/g at a current density of0.2A/g. At4A/g, the specific capacitance is274F/g, and the value maintained200F/g when the electrode was circulated for10000circles, with a retention rate of72.99%. With a current density of10A/g, the specific capacitance decays from249F/g to148F/g after10000cycles.The carbon products obtained under a cell voltage of4.5V at450?exhibit hierarchical pore structure (abundant micropores with the size of0.6?0.7nm, mesopores with the size of2?2.5nm,3?4nm and5?50nm) and excellent conductivity of0.28?0.49S/cm. FTIR tests demonstrated that oxygen-containing functional groups existed in the carbon product, possibly due to the active dangling bond on the carbon surface. All these results show that, carbon powders obtained at450?possess excellent capacitive performance, namely, high energy density, high power density and excellent cycle performance, indicating a good carbon material for capacitor application. |
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